1. Field of the Invention
The present invention relates to methods of discharge control for rechargeable batteries, to a cordless power tool system adapted to provide over-discharge protection and discharge control for an attached battery pack, and to a battery pack including discharge control and over-discharge protection circuits and components.
2. Description of Related Art
Over the past few years, lithium-ion (Li-ion) batteries have begun replacing nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lead-acid batteries in low-voltage, portable electronic devices such as notebook-type personal computers. As compared to NiCd and NiMH batteries, Li-ion batteries are lighter but have a larger capacity per unit volume. For this reason, the Li-ion batteries have been typically suitable to low-voltage devices that are preferably light and which are required to endure continuous use for a long time. In an over-discharged state, however, the Li-ion batteries deteriorate rapidly, thus Li-ion batteries require over-discharge protection.
A battery pack used in a portable electronic device typically has a plurality of battery cells connected in series. The maximum number of battery cells connected in series in one battery pack is determined by the relationship between the output voltage of the battery pack and a power source voltage supplied from outside at the time of charging. For instance, the typical output voltage of one NiCd battery cell or one NiMH battery cell is 1.2 V, and the power source voltage supplied at the time of charging is approximately 1.7 V. Assuming that an 18V output voltage from a battery pack is suitable for most general purpose electronic devices, the maximum number of NiCd or NiMH battery cells connected in series in the battery pack is 15. On the other hand, the typical output voltage of one Li-ion battery cell is approximately 3.6 V. Accordingly, the maximum number of Li-ion battery cells connected in series in one fictional 18V Li-ion battery pack would be 5.
Unlike a NiCd battery pack and a NiMH battery pack, the Li-ion battery pack may include functionality to protect against fault conditions inside and outside the Li-ion battery pack. This prevents cells in the Li-ion battery pack from deteriorating and shortening useful life of the pack. For instance, if a fault condition such as short-circuiting occurs inside or outside the Li-ion battery pack, a fuse may be provided to cut off an over-discharging current or an overcharging current, if the discharging current or charging current becomes larger than a given current level.
Charge/discharge control and over-discharge protection for secondary batteries such as Li-ion batteries may be described in U.S. Pat. No. 6,492,791 to Saeki et al. FIG. 1 is a block diagram of a prior art battery unit from the '791 patent. The battery unit 1 is mounted on an electronic device 11 and supplies power to the electronic device 11. The battery unit 1 includes battery cells E1, E2, and E3, a voltage monitor circuit 101, a fuse 102, p-channel Field Effect Transistors (FETs) 103 and 104, and power supply terminals 105 and 106.
The electronic device 11 includes a DC-DC converter 12, a device main body 13, a voltage monitor circuit 14, a regulator 15, a main switch 16, and a reset switch 17. The DC-DC converter 12 is connected to the power source terminal 105 of the battery unit 1, and converts the voltage supplied from the battery unit 1 to a desired voltage. The DC-DC converter 12 is also connected to the regulator 15, and converts the voltage supplied from the regulator 15 to a desired voltage.
The voltage converted by the DC-DC converter 12 is supplied to the device main body 13 via the main switch 16. The main switch 16 is turned on to supply the voltage converted by the DC-DC converter 12 to the device main body 13. The main switch 16 is interlocked with the reset switch 17, so that when the main switch 16 is turned on, the reset switch 17 is also turned on.
FIG. 2 is a circuit diagram of a voltage monitor circuit for the prior art battery unit of FIG. 1. As shown in FIG. 2, the voltage monitor circuit 101 comprises an overcharge monitor circuit 101a and an over-discharge monitor circuit 101b. The overcharge monitor circuit 101a monitors whether the battery cells E1, E2, and E3 are in an overcharged state, and switches off the FET 103 when the battery cells are in an overcharged state. The over-discharge monitor circuit 101b monitors whether the battery cells E1, E2, and E3 are in an over-discharged state, and switches off the FET 104 when the battery cells E1, E2, and E3 are in an over-discharged state.
The overcharge monitor circuit 101a includes a comparator 121 that compares the voltage of the battery cell E1 with a reference voltage Vref1 generated by a reference power source e1a. If the voltage of the battery cell E1 is higher than the reference voltage Vref1, the comparator 121 outputs “1”. If the voltage of the battery cell E1 is lower than the reference voltage Vref1, the comparator 121 outputs “0”. Here, “1” indicates that the output of a comparator is at the high logic level, and “0” indicates that the output of a comparator is at the low logic level. Similarly, for cell E2, comparator 122 outputs a “1” if voltage of the battery cell E2 is higher than reference voltage Vref1 generated by reference power source e1b, else it outputs a “0”. Further, comparator 123 compares the voltage of battery cell E3, and outputs “1”, or “0”, depending on whether the voltage of battery cell E3 is higher or lower than the reference voltage Vref1 generated by reference power source e1c.
The outputs of the comparators 121, 122, and 123 are subject to an OR operation at an OR gate 124, which supplies a result of the OR operation to the gate of the FET 103. If any of the outputs of the comparators 121, 122, and 123 is “1”, i.e., if any of the battery cells E1, E2, and E3 is in an overcharged state and the signal supplied from the OR gate 124 to the gate of the FET 103 is “1”, the FET 103 is switched off so as to prevent overcharge.
The over-discharge monitor circuit 101b includes a comparator 111 that compares the voltage of the battery cell E1 with a reference voltage Vref2 generated by the reference power source e2a. If the voltage of the battery cell E1 is higher than the reference voltage Vref2, the comparator 111 outputs “0”. If the voltage of the battery cell E1 is lower than the reference voltage Vref2, the comparator 111 outputs “1”. Similarly, comparator 112 compares the voltage of the battery cell E2 with a reference voltage Vref2 generated by the reference power source e2b. If the voltage of the battery cell E2 is higher than the reference voltage Vref2, the comparator 112 outputs “0”. If the voltage of the battery cell E2 is lower than the reference voltage Vref2, the comparator 112 outputs “1”. The functions of comparator 113 are also similar; the comparator 113 outputs “0” if voltage of the E3 cell is higher than Vref2 generated by reference power source e2c, else comparator 113 outputs “1”.
The outputs of the comparators 111, 112, and 113 are subject to an OR operation at an OR gate 114, which supplies a result of the OR operation to the gate of the FET 104. If any of the outputs of the comparators 111, 112, and 113 is “1”, i.e., if any of the battery cells E1, E2, and E3 is in an over-discharged state and the signal supplied from the OR gate 114 to the gate of the FET 104 is “1”, the FET 104 is switched off so as to prevent over-discharge of the battery unit 1.
FIG. 3 is a circuit diagram of the discharge control circuit 2 in the prior art battery unit of FIG. 1. FIG. 3 is provided to illustrate the relation between discharge control circuit 2 and over-discharge monitor circuit 101b. 
The discharge control circuit 2 includes a flip-flop 5 that has a set terminal and a reset terminal. The output of the flip-flop 5 is set at “1” when its set terminal is set at “1”. The output of the flip-flop 5 is reset at “0” when its reset terminal is set at “1”. The set terminal 3 is connected to the set terminal of the flip-flop 5, and the output of an OR gate 6 is supplied to the reset terminal of the flip-flop 5.
The OR gate 6 is supplied with a reset signal applied to the reset terminal 4 and the output of a comparator 8 so as to perform an OR operation on the reset signal and the output of the comparator 8. The comparator 8 detects a voltage between the source and the drain of the charge control FET 103. If the voltage between the source and the drain is higher than a threshold value, the comparator 8 outputs a high-level signal. If the voltage between the source and the drain is lower than the threshold value, the comparator 8 outputs a low-level signal. In this manner, the comparator 8 judges whether the charging voltage is higher than a predetermined level or not from the voltage between the source and the drain of the charge control FET 103, thereby resetting the flip-flop 5.
When the flip-flop 5 is set and the discharge control FET 104 is OFF before charging, the comparator 8 also detects electrification from the voltage between the source and the drain of the charge control FET 103. If electrification is detected, the flip-flop 5 is reset, the output of the flip-flop 5 becomes “low”, and the discharge control FET 104 is turned on.
When the set terminal 3 becomes “1”, the flip-flop 5 outputs “1”. When the output of the reset terminal 4 or the output of the comparator 8 becomes “1”, the flip-flop 5 outputs “0”. The output of the flip-flop 5 is supplied to an OR gate 7. The OR gate 7 is supplied with the output of the over-discharge control circuit 101b as well as the output of the flip-flop 5. The OR gate 7 performs an OR operation on the output of the flip-flop 5 and the output of the over-discharge control circuit 101b. 
The output of the OR gate 7 is supplied to the discharge control FET 104. The discharge control FET 104 is OFF when the output of the OR gate 7 is “1”, and is ON when the output of the OR gate is “0”. In other words, when the flip-flop 5 is set, the discharge control FET 104 becomes “1” and is turned off. When the flip-flop 5 is reset and outputs “0”, the discharge control FET 104 is turned on or off depending on the output of the over-discharge control circuit 101b. 
The above-described battery unit with charge/discharge control and over-discharge protection is designed primarily for low-voltage portable electronic devices such as notebook-type personal computers, cellular phones, etc., which require voltage generally on the order of 2 to 4 volts. Such devices are characterized by using battery packs composed of cells (such as Li-ion, NiCd, NiMH cells) that provide a maximum output voltage of about 4.2 volts/cell.
However, much higher voltages than described above are required for higher-power electronic devices such as cordless power tools. Accordingly, higher-power battery packs may be in the process of being developed for cordless power tools. Such “high-power” battery packs may provide higher voltage outputs than conventional NiCd and NiMH battery packs (and substantially higher power than conventional Li-ion packs used for PCs and cell phones), and at a much reduced weight (as compared to conventional NiCd or NiMH battery packs used as power sources in conventional cordless power tools). A characteristic of these battery packs is that the battery packs may exhibit substantially lower impedance characteristics than conventional NiCd, NiMH and/or even the lower power Li-ion packs.
However, as this battery technology advances the introduction of lower impedance chemistries (such as lithium-ion chemistry) and construction styles to develop secondary batteries generating substantially higher output voltages then about 4.2 volts/cell may possibly create compatibility issues with existing cordless power tools. As total internal pack impedance drops, the pack can supply substantially higher current to an attached electronic component, such as a power tool. As current through a tool motor of the attached tool increases, demagnetization forces (e.g., the number of armature turns of the motor times the current, ampere-turns) could substantially increase beyond a desired or design limit in the motor. Such undesirable demagnetization could thus potentially burn up the tool motor.
For example, a lower impedance electrical source could cause damage to a tool's motor when the tool is held at stall condition. During motor stall, the motor and battery impedances are the only mechanisms to limit the current since there is no back-EMF created by the motor. With a lower impedance pack, the currents would be higher. Higher currents through the motor might cause a stronger de-magnetization force than what the tool's permanent magnets were designed to withstand. Additionally, start-up of the tool could produce excessive starting currents and cause demagnetization of the motor. Thermal overload could also be a result of using a low impedance electrical source in an existing power tool, as the new batteries may be designed to run longer and harder than what the original cordless tool system was designed. Accordingly, over-discharge and/or other current limiting controls may need to be in place before these developing lower-impedance batteries may be used with existing cordless power tools, for example.